Mesoporous molecular sieves modified with carbonaceous deposits

There were investigated aluminosilicate MCM-41 samples in the as-prepared form and those modified by the deposition of carbonaceous compounds during conversion of cyclohexene for 12 h. The amount of the deposits decreased with the rising reaction temperature and increased with the Al content of the samples. The cyclohexene conversion followed mainly two mechanisms: cyclohexene skeletal isomerization and hydrogen transfer. The products with 6 carbon atoms in a molecule prevailed in all cases. The process of conversion, proceeding on the Bronsted acid sites, resulted in formation of both coke deposits and volatile products. The creation of coke caused a decrease in the effective concentration of both the Bronsted and the Lewis acid sites. Thermodesorption of pyridine showed that (i) the concentration of these sites before and after the conversion differed only slightly and (ii) the acidic strength of the Bronsted sites was practically independent of their concentration and the sample Si/Al ratio. The chemical composition of the deposits was insignificantly affected by the Al content of the materials and depended mostly on the temperature and duration of the reaction. At relatively low temperatures, both aliphatic and aromatic compounds were formed, being rather weakly bound to the surface of the material. After a longer (55 h) reaction period, some deposits appeared that were strongly bound to the surface. Isotherms of adsorption of water, benzene, and nitrogen were determined, from which a mechanism of this process was derived. It included most probably multilayer adsorption at lower relative pressures, followed by capillary condensation. The sorption capacities of the uncoked samples for benzene and nitrogen were relatively high and independent of the sample Al content. In the case of water, however, an observed reduction in the sorption capacity with the increasing Al content suggested that clusters of the adsorbed molecules formed around the Al centers and caused partial clogging of the material pores. The deposited coke strongly decreased both the surface area and the sorption capacity of the materials.